The operation of an AC thin film electroluminescent (TFEL) display panel is based on the principle that a luminescent material (e.g., phosphor) will emit light when a voltage of sufficient magnitude is applied across it. The TFEL display is typically constructed with luminescent material sandwiched between a dielectic insulator and a plurality of row electrodes on one side, and a plurality of column electrodes on the opposite side. Each intersection of the plurality of row and column electrodes defines a pixel. A typical high resolution TFEL display panel may have 512 row electrodes and 640 column electrodes, resulting in 327,680 pixels. Commonly assigned U.S. application, Ser. No. 07/897,201, attorney docket number R-3612N, entitled "Low Resistance, Thermally Stable Electrode Structure for Electroluminescent Displays" filed Jun. 11, 1992, discloses the construction of a TFEL display panel.
The luminance of each pixel in the panel is dependent upon the magnitude of the voltage applied across the particular row and column electrode which define the pixel. As a result of this relationship, gray shading can be achieved by controlling the magnitude of the voltage across the pixel. As an example, each pixel may display one of sixteen luminance levels depending on the magnitude of the voltage applied across the pixel. The magnitude of the minimum voltage required across the pixel before the electroluminescent material will display light is often referred to as the threshold voltage.
A problem with a TFEL display panel is that it often suffers from latent imaging problems which cause ghost images on the display panel. This is typically a result of the pixel's voltage-time average being non-zero when averaged over several scans through the panel.
U.S. Pat. No. 4,975,691 to J. Y. Lee entitled "Scan Inversion Symmetric Drive" discusses the problem of latent images and discloses alternating the order the rows are scanned in an attempt to reduce the latent images. More particularly the '691 patent discloses the steps of first applying a refresh pulse of a first polarity (e.g.,-160) to all the rows in the panel, then sequencing through each row of the panel updating the pixels one row at a time. Once all the rows have been addressed, the refresh pulse of the first polarity is applied again for a short duration, and rows are again scanned through but this time in the reverse order of the previous scan. A problem with this approach is that is does not utilize the advantages of gray-scaling and hence is not subject to the problems therewith.
U.S. Pat. No. 4,733,228 to R. T. Flegal entitled "Transformer-Coupled Drive Network For A TFEL Panel" also discloses a symmetric drive scheme for reducing latent image problems. The '228 patent discloses sequentially scanning through all the rows and applying a voltage of a first polarity (e.g., -160 vdc) on the row electrodes, and on the next scan through applying a voltage of a second polarity (e.g., 210 vdc). A modulation voltage is then applied to the column electrodes to control pixel luminance. The symmetric drive is achieved by reversing the polarity of the row driver voltage each frame, and separating the row voltages by an amount equal to the magnitude of the column modulation voltage value. A problem with this approach is that it fails to provide a symmetric drive scheme for a panel employing gray scaling. Another problem is the circuit complexity and cost associated with providing dual polarity row drivers.